Implement TextClassifier.getLogger API

 - Introduces getLogger() API.
 - A logger should run in the client's process. This helps us manage
   sessions specific to a client.
 - The logger exposes a tokenizer that clients may use to tokenize
   strings for logging purposes.
 - Logger subclasses need to provide a writeEvent() implementation.
 - SelectionEvent is serializable over IPC.
 - Logger takes care of the session management. It writes session
   specific information into the SelectionEvent.
 - We still keep the SmartSelectionEventTracker for now so clients
   can slowly move off of it. The plan is to delete it.
 - The plan is to include support other event types. e.g. link events.

Bug: 64914512
Bug: 67609167
Test: See topic
Change-Id: Ic9470cf8f969add8a4c6570f78603d0b118956cd